OBJECTIVE: To determine gender differences in the brain structures of rhesus monkeys, we measured the volume of whole brain, the size of corpus callosum, and amygdala by MRI image analysis methods. RESULTS This study suggests that the whole brain volume and the size of the corpus callosum of young adult female rhesus monkeys are approximately 15% smaller than those of young adult males. Although the brain weight of the female rhesus monkey is lighter than that of the male, regional sexual dimorphism is not well known in the brains of nonhuman primates (Cupp and Uemura, 1981). Using a method of MRI image analysis, we measured the whole brain volume and the size of the corpus callosum (mid-sagittal) and amygdala (coronal section) in 41 MRI images of juvenile to adult rhesus monkeys from 8 months to 7.2 years of age. For the brain volume, 14 MRI images (T-1 weighted, 1.3 mm thick, 60 serial coronal images) of 5 male and 3 female monkeys,1.5 to 5.1 years of age, were used. The sizes of the corpus callosum and amygdala were measured in 27 MRI images (T-1 w., 3 mm thick, sagittal and coronal images) of 12 male and 15 female monkeys, 8 months to 7.2 years of age. The results revealed that the brain volume of male monkeys slightly increased with age; 80.5 (cm3) at 1.5 years and 94.9 at 5.1 years. The brain volume of female monkeys was unchanged from the age of 1.5 to 4 years; the range was 70.9 to 74.8. By the age of 4 years, the average volume of the male brain was 88.2 q 1.9 (SE) and that of the female brain 72.2 q 0.7 cm3. The average area of the corpus callosum increases from 8 months to 3 years; 0.56 (cm2) to 0.93 in males and 0.45 to 0.66 in females. The average area of the amygdala after the age of 8 months in both genders was unchanged; 1.08 q 0.03 cm2. We have done MRI imaging of the brain of 4 pseudohermaphrodite female monkeys and the image anaysis of these monkeys will be compared to the above brain structures of age-matched normal female monkeys. FUTURE DIRECTIONS Based on the above data, we are planning to compare the structural differences of the brain between normal adult female and pseudohermaphrodite female monkeys who received testosterone during early fetal period. KEY WORDS MRI, brain structure, gender difference, rhesus monkey